Observation of Moment-Dependent and Field-Driven Unidirectional Magnetoresistance in CoFeB/InSb/CdTe Heterostructures.

Magnetoresistance effects are crucial for understanding the charge-spin transport as well as propelling the advancement of spintronic applications. Here, we report the coexistence of magnetic-moment-dependent (MD) and magnetic-field-driven (FD) unidirectional magnetoresistance (UMR) effects in CoFeB/InSb/CdTe heterostructures. The strong spin-orbital coupling of InSb and the matched impedance at the CoFeB/InSb interface warrant a distinct MD-UMR effect at room temperature, while the interaction between the in-plane magnetic field and the Rashba effect at the InSb/CdTe interface induces the marked FD-UMR signal that dominates the high-field region. Moreover, owning to different spin scattering mechanisms, these two types of non-reciprocal charge transports show opposite polarities with respect to the magnetic field direction, which further enables an effective phase modulation of the angular-dependent magnetoresistance. The demonstration of the tunable UMR response validates our CoFeB/InSb/CdTe system as a suitable integrated building block for multifunctional spintronic memory and sensor designs.

Anomalous hybridized excitons induced by combined effects of Van der Waals coupling and Rashba spin-orbit coupling.

As a typical transition-metal dichalcogenide, MoS2has drawn wide attention due to its good stability and excellent physicochemical properties, making it suitable for visible-region optoelectronic devices. To expand its application, bandgap engineering via heterostructure, thus far, was conventionally employed to tune the band gap. However, this strategy has the disadvantage that energy levels of bands do not show obvious changes compared to the isolated components, limiting the range of applications. Here, we achieve hybridized excitons induced by combined effects of Van der Waals (vdW) coupling and Rashba spin-orbit coupling (SOC), with a small exciton energy of 0.65 eV. For this purpose, we design a MoS2/MoWC heterostructure, where a built-in field (due to the absence of mirror symmetry) induces the Rashba SOC and contributes to the anomalous hybridized states, combined with the vdW coupling. An effective model is proposed to demonstrate the anomalous hybridized states for the heterostructure. Our approach reveals a novel mechanics model for hybridized excitons states, providing new physical ways to achieve infrared-region devices.

Nonvolatile Electric Control of Rashba Spin Splitting in Sb/In2Se3 Heterostructure.

Ferroelectric Rashba semiconductors (FRS) are highly demanded for their potential capability for nonvolatile electric control of electron spins. An ideal FRS is characterized by a combination of room temperature ferroelectricity and a strong Rashba effect, which has, however, been rarely reported. Herein, we designed a room-temperature FRS by vertically stacking a Sb monolayer on a room-temperature ferroelectric In2Se3 monolayer. Our first-principles calculations reveal that the Sb/In2Se3 heterostructure exhibits a clean Rashba splitting band near the Fermi level and a strong Rashba effect coupled to the ferroelectric order. Switching the electric polarization direction enhances the Rashba effect, and the flipping is feasible with a low energy barrier of 22 meV. This Rashba-ferroelectricity coupling effect is robust against changes of the heterostructure interfacial distance and external electric fields. Such a nonvolatile electrically tunable Rashba effect at room temperature enables potential applications in next-generation data storage and logic devices operated under small electrical currents.

Identification of Semiconductor Nanocrystals with Bright Ground-State Excitons.

While semiconductor nanocrystals provide versatile fluorescent materials for light-emitting devices, their brightness suffers from the "dark exciton"─an optically inactive electronic state into which nanocrystals relax before emitting. Recently, a theoretical mechanism, the Rashba effect, was discovered that can overcome this limitation by inverting the lowest-lying levels and creating a bright excitonic ground state. However, no methodology is available to systematically identify materials that exhibit this inversion, hindering the development of superbright nanocrystals and their devices. Here, based on a detailed understanding of the Rashba mechanism, we demonstrate a procedure that reveals previously unknown "bright-exciton" nanocrystals. We first define physical criteria to reduce over 500,000 known solids to 173 targets. Higher-level first-principles calculations then refine this list to 28 candidates. From these, we select five with high oscillator strength and develop effective-mass models to determine the nature of their lowest excitonic state. We confirm that four of the five solids yield bright ground-state excitons in nanocrystals. Thus, our results provide a badly needed roadmap for experimental investigation of bright-exciton nanomaterials.

Optical Studies of Doped Two-Dimensional Lead Halide Perovskites: Evidence for Rashba-Split Branches in the Conduction Band.

Two-dimensional (2D) hybrid organic/inorganic perovskites are an emerging materials class for optoelectronic and spintronic applications due to strong excitonic absorption and emission, large spin-orbit coupling, and Rashba spin-splitting effects. For many of the envisioned applications, tuning the majority charge carrier (electron or hole) concentration is desirable, but electronic doping of metal-halide perovskites has proven to be challenging. Here, we demonstrate electron injection into the lower-energy branch of the Rashba-split conduction band of 2D phenethylammonium lead iodide by means of n-type molecular doping at room temperature. The molecular dopant, benzyl viologen (BV), is shown to compensate adventitious p-type impurities and can lead to a tunable Fermi level above the conduction band minimum and increased conductivity in intrinsic samples. The doping-induced carrier concentration is monitored by the observation of free-carrier absorption and intraband optical transitions in the infrared spectral range. These optical measurements allow for an estimation of the Rashba splitting energy ER ≈38 ± 4 meV. Photoinduced quantum beating measurements demonstrate that the excess electron density reduces the electron spin g-factor by ca. 6%. This work demonstrates controllable carrier concentrations in hybrid organic/inorganic perovskites and yields potential for room temperature spin control through the Rashba effect.

Circularly Polarized Luminescence Without External Magnetic Fields from Individual CsPbBr3 Perovskite Quantum Dots.

Lead halide perovskite quantum dots (QDs), the latest generation of the colloidal QD family, exhibit outstanding optical properties, which are now exploited as both classical and quantum light sources. Most of their rather exceptional properties are related to the peculiar exciton fine-structure of band-edge states, which can support unique bright triplet excitons. The degeneracy of the bright triplet excitons is lifted with energetic splitting in the order of millielectronvolts, which can be resolved by the photoluminescence (PL) measurements of single QDs at cryogenic temperatures. Each bright exciton fine-structure-state (FSS) exhibits a dominantly linear polarization, in line with several theoretical models based on the sole crystal field, exchange interaction, and shape anisotropy. Here, we show that in addition to a high degree of linear polarization, the individual exciton FSS can exhibit a non-negligible degree of circular polarization even without external magnetic fields by investigating the four Stokes parameters of the exciton fine-structure in individual CsPbBr3 QDs through Stokes polarimetric measurements. We observe a degree of circular polarization up to ∼38%, which could not be detected by using the conventional polarimetric technique. In addition, we found a consistent transition from left- to right-hand circular polarization within the fine-structure triplet manifold, which was observed in magnetic-field-dependent experiments. Our optical investigation provides deeper insights into the nature of the exciton fine structures and thereby drives the yet-incomplete understanding of the unique photophysical properties of this class of QDs for the benefit of future applications in chiral quantum optics.

Unusual Dzyaloshinskii-Moriya Interaction in Graphene/Fe3GeTe2 Van der Waals Heterostructure.

Dzyaloshinskii-Moriya interaction (DMI) is shown to induce a topologically protected chiral spin texture in magnetic/nonmagnetic heterostructures. In the context of van der Waals spintronic devices, graphene emerges as an excellent candidate material. However, due to its negligible spin-orbit interaction, inducing DMI to stabilize topological spins when coupled to 3d-ferromagnets remains challenging. Here, it is demonstrated that, despite these challenges, a sizeable Rashba-type spin splitting followed by significant DMI is induced in graphene/Fe3GeTe2. This is made possible due to an interfacial electric field driven by charge asymmetry together with the broken inversion symmetry of the heterostructure. These findings reveal that the enhanced DMI energy parameter, resulting from a large effective electron mass in Fe3GeTe2, remarkably contributes to stabilizing non-collinear spins below the Curie temperature, overcoming the magnetic anisotropy energy. These results are supported by the topological Hall effect, which coexists with the non-trivial breakdown of Fermi liquid behavior, confirming the interplay between spins and non-trivial topology. This work paves the way toward the design and control of interface-driven skyrmion-based devices.

Topology-Driven Coulomb Drag in van der Waals Heterostructure with Broken Inversion Symmetry.

As Coulomb drag near charge neutrality (CN) is driven by fluctuations or inhomogeneity in charge density, the topology should play an extremely important role. Interlinking Coulomb drag and topology could reveal how the system's nontrivial topology influences the electron-electron interactions at the quantum level. However, such an aspect is overlooked as most studies focus on symmetric drag systems without topology. To understand this topological aspect, we need to study Coulomb drag in an asymmetric system with a broken inversion symmetry and strong spin-orbit coupling (SOC). Here we experimentally demonstrate the energy-driven Coulomb drag in an asymmetric van der Waals heterostructure composed of black phosphorus and rhenium disulfide characterized by broken inversion symmetry. Temperature-dependent transport measurements near CN provide compelling evidence for the energy-driven Coulomb drag due to electron-hole coupling that is energetically favored in a broken-gap heterojunction, as confirmed by Hall coefficient sign reversal with temperature. Moreover, contrary to the symmetric devices, our results exhibit magnetic-field-free, i.e., topology-driven, Hall drag, revealing an intrinsic coupling between energy and charge modes. This is the manifestation of nonzero Berry curvature, akin to a magnetic field in momentum space, in a Rashba system, which arises from the SOC and broken inversion symmetry of the heterostructure.

Rashba-like Spin Textures in Graphene Promoted by Ferromagnet-Mediated Electronic Hybridization with a Heavy Metal.

Epitaxial graphene/ferromagnetic metal (Gr/FM) heterostructures deposited onto heavy metals have been proposed for the realization of spintronic devices because of their perpendicular magnetic anisotropy and sizable Dzyaloshinskii-Moriya interaction (DMI), allowing for both enhanced thermal stability and stabilization of chiral spin textures. However, establishing routes toward this goal requires the fundamental understanding of the microscopic origin of their unusual properties. Here, we elucidate the nature of the induced spin-orbit coupling (SOC) at Gr/Co interfaces on Ir. Through spin- and angle-resolved photoemission spectroscopy along with density functional theory, we show that the interaction of the heavy metals with the Gr layer via hybridization with the FM is the source of strong SOC in the Gr layer. Furthermore, our studies on ultrathin Co films underneath Gr reveal an energy splitting of ∼100 meV for in-plane and negligible for out-of-plane spin polarized Gr π-bands, consistent with a Rashba-SOC at the Gr/Co interface, which is either the fingerprint or the origin of the DMI. This mechanism vanishes at large Co thicknesses, where neither in-plane nor out-of-plane spin-orbit splitting is observed, indicating that Gr π-states are electronically decoupled from the heavy metal. The present findings are important for future applications of Gr-based heterostructures in spintronic devices.

Spin-Orbit Torque Vector Quantification in Nanoscale Magnetic Tunnel Junctions.

Spin-orbit torques (SOT) allow ultrafast, energy-efficient toggling of magnetization state by an in-plane charge current for applications such as magnetic random-access memory (SOT-MRAM). Tailoring the SOT vector comprising of antidamping (TAD) and fieldlike (TFL) torques could lead to faster, more reliable, and low-power SOT-MRAM. Here, we establish a method to quantify the longitudinal (TAD) and transverse (TFL) components of the SOT vector and its efficiency χAD and χFL, respectively, in nanoscale three-terminal SOT magnetic tunnel junctions (SOT-MTJ). Modulation of nucleation or switching field (BSF) for magnetization reversal by SOT effective fields (BSOT) leads to the modification of SOT-MTJ hysteresis loop behavior from which χAD and χFL are quantified. Surprisingly, in nanoscale W/CoFeB SOT-MTJ, we find χFL to be (i) twice as large as χAD and (ii) 6 times as large as χFL in micrometer-sized W/CoFeB Hall-bar devices. Our quantification is supported by micromagnetic and macrospin simulations which reproduce experimental SOT-MTJ Stoner-Wohlfarth astroid behavior only for χFL > χAD. Additionally, from the threshold current for current-induced magnetization switching with a transverse magnetic field, we show that in SOT-MTJ, TFL plays a more prominent role in magnetization dynamics than TAD. Due to SOT-MRAM geometry and nanodimensionality, the potential role of nonlocal spin Hall spin current accumulated adjacent to the SOT-MTJ in the mediation of TFL and χFL amplification merits to be explored.

Research Progress on Rashba Effect in Two-Dimensional Organic-Inorganic Hybrid Lead Halide Perovskites.

The Rashba effect appears in the semiconductors with an inversion-asymmetric structure and strong spin-orbit coupling, which splits the spin-degenerated band into two sub-bands with opposite spin states. The Rashba effect can not only be used to regulate carrier relaxations, thereby improving the performance of photoelectric devices, but also used to expand the applications of semiconductors in spintronics. In this mini-review, recent research progress on the Rashba effect of two-dimensional (2D) organic-inorganic hybrid perovskites is summarized. The origin and magnitude of Rashba spin splitting, layer-dependent Rashba band splitting of 2D perovskites, the Rashba effect in 2D perovskite quantum dots, a 2D/3D perovskite composite, and 2D-perovskites-based van der Waals heterostructures are discussed. Moreover, applications of the 2D Rashba effect in circularly polarized light detection are reviewed. Finally, future research to modulate the Rashba strength in 2D perovskites is prospected, which is conceived to promote the optoelectronic and spintronic applications of 2D perovskites.

Emerging Nonlinear Photocurrents in Lead Halide Perovskites for Spintronics.

Lead halide perovskites (LHPs) containing organic parts are emerging optoelectronic materials with a wide range of applications thanks to their high optical absorption, carrier mobility, and easy preparation methods. They possess spin-dependent properties, such as strong spin-orbit coupling (SOC), and are promising for spintronics. The Rashba effect in LHPs can be manipulated by a magnetic field and a polarized light field. Considering the surfaces and interfaces of LHPs, light polarization-dependent optoelectronics of LHPs has attracted attention, especially in terms of spin-dependent photocurrents (SDPs). Currently, there are intense efforts being made in the identification and separation of SDPs and spin-to-charge interconversion in LHP. Here, we provide a comprehensive review of second-order nonlinear photocurrents in LHP in regard to spintronics. First, a detailed background on Rashba SOC and its related effects (including the inverse Rashba-Edelstein effect) is given. Subsequently, nonlinear photo-induced effects leading to SDPs are presented. Then, SDPs due to the photo-induced inverse spin Hall effect and the circular photogalvanic effect, together with photocurrent due to the photon drag effect, are compared. This is followed by the main focus of nonlinear photocurrents in LHPs containing organic parts, starting from fundamentals related to spin-dependent optoelectronics. Finally, we conclude with a brief summary and future prospects.

Strong Sliding Ferroelectricity and Interlayer Sliding Controllable Spintronic Effect in Two-Dimensional HgI2 Layers.

Exploration of two-dimensional (2D) sliding ferroelectric (FE) materials with experimentally detectable ferroelectricity and value-added novel functionalities is highly sought for the development of 2D "slidetronics". Herein, based on first-principles calculations, we identify the synthesizable van der Waals (vdW) layered crystals HgX2 (X = Br and I) as a new class of 2D sliding ferroelectrics. Both HgBr2 and HgI2 in 2D multilayered forms adopt the preferential stacking sequence, leading to room temperature stable out-of-plane (vertical) ferroelectricity that can be reversed via the sliding of adjacent monolayers. Owing to strong interlayer coupling and interfacial charge rearrangement, 2D HgI2 layers possess strong sliding ferroelectricity up to 0.16 µC/cm2, readily detectable in experiment. Moreover, robust sliding ferroelectricity and interlayer sliding controllable Rashba spin texture of FE-HgI2 layers enable potential applications as 2D spintronic devices such that the electric control of electron spin detection can be realized at the 2D regime.

Highly Efficient Spintronic Terahertz Emitter Utilizing a Large Spin Hall Conductivity of Type-II Dirac Semimetal PtTe2.

The remarkable spin-charge interconversion ability of transition metal dichalcogenides (TMDs) makes them promising candidates for spintronic applications. Nevertheless, their potential as spintronic terahertz (THz) emitters (STEs) remains constrained mainly due to their sizable resistivity and low spin Hall conductivity (SHC), which consequently result in modest THz emission. In this work, the TMD PtTe2, a type-II Dirac semimetal is effectively utilized to develop efficient STEs. This high efficiency primarily results from the large SHC of PtTe2, stemming from its low resistivity and significant spin-to-charge conversion efficiency, attributed to surface states and the local Rashba effect in addition to the inverse spin Hall effect. Remarkably, the peak THz emission from PtTe2/Co-STE exceeds that of Pt/Co-STE by ∼15% and is nearly double that of a similarly thick Pt/Co-STE. The efficient THz emission in the PtTe2/Co heterostructure opens new possibilities for utilizing the semimetal TMDs for developing THz emitters.

Visualizing the Low-Energy Electronic Structure of Prototypical Hybrid Halide Perovskite through Clear Band Measurements.

Organic-inorganic hybrid perovskites (OIHPs) are a promising class of materials that rival conventional semiconductors in various optoelectronic applications. However, unraveling the precise nature of their low-energy electronic structures continues to pose a significant challenge, primarily due to the absence of clear band measurements. Here, we investigate the low-energy electronic structure of CH3NH3PbI3 (MAPI3) using angle-resolved photoelectron spectroscopy combined with ab initio density functional theory. We successfully visualize the electronic structure of MAPI3 near the bulk valence band maximum by using a laboratory photon source (He Iα, 21.2 eV) at low temperature and explore its fundamental properties. The observed valence band exhibits a highly isotropic and parabolic band characterized by small effective masses of 0.20-0.21 me, without notable spectral signatures associated with a large polaron or the Rashba effect, subjects that are intensely debated in the literature. Concurrently, our spin-resolved measurements directly disprove the giant Rashba scenario previously suggested in a similar perovskite compound by establishing an upper limit for the Rashba parameter (αR) of 0.28 eV Å. Our results unveil the unusually complex nature of the low-energy electronic structure of OIHPs, thereby advancing our fundamental understanding of this important class of materials.

Strain engineering of Zeeman and Rashba effects in transition metal dichalcogenide nanotubes and their Janus variants: anab initiostudy.

We study the influence of mechanical deformations on the Zeeman and Rashba effects in transition metal dichalcogenide nanotubes and their Janus variants from first principles. In particular, we perform symmetry-adapted density functional theory simulations with spin-orbit coupling to determine the variation in the electronic band structure splittings with axial and torsional deformations. We find significant effects in molybdenum and tungsten nanotubes, for which the Zeeman splitting decreases with increase in strain, going to zero for large enough tensile/shear strains, while the Rashba splitting coefficient increases linearly with shear strain, while being zero for all tensile strains, a consequence of the inversion symmetry remaining unbroken. In addition, the Zeeman splitting is relatively unaffected by nanotube diameter, whereas the Rashba coefficient decreases with increase in diameter. Overall, mechanical deformations represent a powerful tool for spintronics in nanotubes.

Persistence of Structural Distortion and Bulk Band Rashba Splitting in SnTe above Its Ferroelectric Critical Temperature.

The ferroelectric semiconductor α-SnTe has been regarded as a topological crystalline insulator, and the dispersion of its surface states has been intensively measured with angle-resolved photoemission spectroscopy (ARPES) over the past decade. However, much less attention has been given to the impact of the ferroelectric transition on its electronic structure, and in particular on its bulk states. Here, we investigate the low-energy electronic structure of α-SnTe with ARPES and follow the evolution of the bulk-state Rashba splitting as a function of temperature, across its ferroelectric critical temperature of about Tc ≈ 110 K. Unexpectedly, we observe a persistent band splitting up to room temperature, which is consistent with an order-disorder contribution of local dipoles to the phase transition that requires the presence of fluctuating dipoles above Tc. We conclude that no topological surface state can occur under these conditions at the (111) surface of SnTe, at odds with recent literature.

Current-Induced Magnetization Switching in Light-Metal-Oxide/Ferromagnetic-Metal Bilayers via Orbital Rashba Effect.

The orbital angular momentum (OAM) generation as well as its associated orbital torque is currently a matter of great interest in spin-orbitronics and is receiving increasing attention. In particular, recent theoretical work predicts that the oxidized light metal Cu can serve as an efficient OAM generator through its surface orbital Rashba effect. Here, for the first time, the crucial current-induced magnetic-field-free in-plane magnetization reversal is experimentally demonstrated in CoFeB/CuOx bilayers without any heavy elements. We show that the critical current density can be comparable to that of strong spin-orbit coupling systems with heavy metals (Pt and Ta) and that the magnetization reversal mechanism is governed by coherent rotation in the grains through the second-harmonic and magneto-optical Kerr effect measurements. Our results indicate that light metal oxides can play an equally important role as heavy metals in magnetization reversal, broadening the choice of materials for engineering spintronic devices.

Rashba spin-splitting in Janus SnXY/WXY (X, Y = S, Se, Te; X ≠ Y) heterostructures.

Janus transition metal dichalcogenide monolayers have shown a lack of mirror symmetry perpendicular to the 2D plane. The breaking of out-of-plane symmetry, along with the spin-orbit coupling, induces Rashba spin-splitting (RSS) in these materials. In this work, RSS in Janus tin dichalcogenide monolayers are studied. In addition, the heterostructures (HSs) of Janus SnXY and WXY (X, Y = S, Se, Te; X ≠ Y) monolayers are discussed. A RSS energy of about 43 meV, more significant than the room temperature energy, is observed in the Janus SnSSe/WSSe HS. The consequences of vertical strain on the semiconducting HS are examined. Compressive vertical strain enhances and tensile strain reduces, the spin-splitting. For the compressive strain of 10.4%, Janus SnSSe/WSSe HS remains semiconductor with only Rashba bands surrounding near the Fermi level. Enhanced Rashba parameter of about 0.96 eV Å and splitting energy of about 72 meV are observed. These findings confirm that Janus SnSSe/WSSe HS is a productive Rashba material for spintronic device applications.

Enhanced Superconductivity and Rashba Effect in a Buckled Plumbene-Au Kagome Superstructure.

Plumbene, with a structure similar to graphene, is expected to possess a strong spin-orbit coupling and thus enhances its superconducting critical temperature (Tc ). In this work, a buckled plumbene-Au Kagome superstructure grown by depositing Au on Pb(111) is investigated. The superconducting gap monitored by temperature-dependent scanning tunneling microscopy/spectroscopy shows that the buckled plumbene-Au Kagome superstructure not only has an enhanced Tc with respect to that of a monolayer Pb but also possesses a higher value than what owned by a bulk Pb substrate. By combining angle-resolved photoemission spectroscopy with density functional theory, the monolayer Au-intercalated low-buckled plumbene sandwiched between the top Au Kagome layer and the bottom Pb(111) substrate is confirmed and the electron-phonon coupling-enhanced superconductivity is revealed. This work demonstrates that a buckled plumbene-Au Kagome superstructure can enhance superconducting Tc and Rashba effect, effectively triggering the novel properties of a plumbene.